Recently, in addition to noise and air pollution, malodor became an issue of public nuisance. Conventionally, the intensity of odor is measured through instrumental analysis or in three bag odor comparison method described later.
In the three bag odor comparison method, three bags are filled with odorless air. Odorless air is prepared by activated carbon treatment of standard air, for example, air on Hirugano plateau, which is famous for its clean atmosphere in Japan. Subsequently, sample air is mixed into one of the three bags. Each air filling these three bags is smelled by six panelists, respectively, to see which bag smells. The concentration of sample air to be mixed in the bag is gradually decreased, until only 50% or less of the panelists have the right answer. By the concentration of sample air at the moment, the intensity of odor, i.e. odor concentration is measured. In the three bag odor comparison method, the odor concentration is measured in the same way for various types of odor.
In the three bag odor comparison method, however, the resulting odor concentration cannot be an objective index because persons have individual variation in their perception of odor. If they smell the same odor many times in one day, their perception to the odor will often vary between the first time and the last time. Also through the instrumental analysis, odor concentration cannot be easily measured, because sensor outputs differ in properties from one another with substances to be measured.
Recently, an odor sensor was developed using a ceramic sensor sensitive to oxidizing gas or reducing gas (refer to Electronics Ceramics July 1986, pages 21-24). The correspondence of the odor sensor's sensitivity to sample air to the dilution scale factor resulting from the aforementioned three bag odor comparison method varies with odor types and odor producing substances. No pure coincidence is seen between the sensor's sensitivity and the dilution scale factor by panelists. Some odor hardly changes even if diluted and other odor changes largely when diluted.